Conventionally, a technique of providing mobility of the entire network composed of a movable terminal group and routers has been examined in IETF NEMO-WG.
NEMO (Network Mobility) described in Network Mobility Basic Support Protocol (written by Vijay Devarapalli et al., rfc3963.txt) is a technique that implements data reachability in a network (referred to as “mobile network” hereinafter) composed of a movable terminal group and mobile routers connecting to an external network such as the Internet by means of the mobile network and a home agent that manages the position of the mobile network.
In such a mobile network system composed of a mobile network and home agent, one mobile network including plural mobile routers may connect to another network through plural nodes. Japanese Patent Unexamined Publication No. 2004-120322 discloses a technique that determines whether or not communication is performed with a mobile router from among plural mobile routers as a node and that manages the communication.
FIG. 18 illustrates the makeup of a conventional mobile network system described in the publication.
In FIG. 18, mobile network 1301 is connected to first subnetwork 1201 through first mobile router (MR) 1311. At this moment, second mobile router (MR) 1312 detects that it has entered a state connectable to second subnetwork 1202. Then, mobile router (MR) 1312 inquires of position management device 1320 in mobile network 1301 whether or not the connection to a subnetwork is switched from first mobile router 1311 to second mobile router 1312. This process allows position management device 1320 to select a mobile router to be an optimum node.
FIG. 17 is a sequence diagram illustrating a communication state in a case where plural mobile routers that have respectively formed different mobile networks exist in one mobile network. Here, a black circle in the diagram indicates a transmission source, and a black triangle indicates a relay node.
In FIG. 17, mobile routers (MR) 1311, 1312 composing mobile network 1301 used to form different mobile networks respectively. Accordingly, they have respectively unique prefixes (network identifier) MNP2, MNP3 being set (step S101).
Here, mobile routers 1311, 1312 connect to access router 1211 and acquires care of addresses CoA_ MR 4, CoA_MR 5, respectively (step S102).
Next, mobile routers 1311, 1312 register MNP2, MNP3 in home agent (HA) 1231 (step S103).
Mobile router 1311 transmits a router advertisement message including MNP2 after completing position registration (step S104), and terminal 1310-1 create an address using MNP2 (step S105).
After that, terminal 1310-1 communicates with external terminal 1221 through mobile router 1311 and home agent 1231 (step S106).
Mobile router 1312, like mobile router 1311, transmits a router advertisement message including MNP3 (step S107).
Terminal 1310-n in mobile network 1301 creates an address using MNP3 in the router advertisement preliminarily received, and communicates (step S108).
This means that mobile network 1301 can include both terminal 1310-1 with an address created from MNP2 and terminal 1310-n with MNP3. They used to communicate with external terminal 1221 through mobile router 1311 and mobile router 1312, respectively.
However, the above-described conventional makeup can be implemented provided that first mobile router 1311 and second mobile router 1312 in FIG. 18 have prefixes with the same internal link from the first. That is, first mobile router 1311 and second mobile router 1312 can relay communication between subordinate terminals 1310-1 to 1310-n, all connected to the internal link, and external terminal 1221. However, if second mobile router 1312 joins a mobile network formed by first mobile router 1311 while another mobile network is formed by second mobile router 1312, for example, second mobile router 1312 cannot transfer a packet from external terminal 1221 to subordinate terminals 1310-1 to 1310-n.
In FIG. 17, meanwhile, when a mobile network having another mobile network with a different prefix connected becomes unable to connect to an external network, due to mobile router 1311 having moved outside the communication area (step S109), terminal 1310-1 communicating through mobile router 1311 cannot continue communication (step S110).
This is because home agent 1231 does not transmit to mobile router 1312, a packet to the address of terminal 1310-1, created from MNP2, as a result that mobile router 1312 has registered MNP3 in home agent 1231.